docs: def prox_gradf12(x, step, j=None, Xs=None):
      """1D gradient operator for x or y"""
      if j == 0:
          return x - step*grad_fx(Xs[0][0], Xs[1][0])
      if j == 1:
          y = x
          return y - step*grad_fy(Xs[0][0], Xs[1][0])
      raise NotImplementedError